Since the introduction of the first microprocessor in the early 1970's, the increased demand for faster and smaller digital processing elements has spurred the rapid growth of digital technology. Large thrusts were made to make this industry both innovative and economical. Today, there are hosts of devices that operate in the digital domain: static and dynamic memories, EPROM's, PAL's, microprocessors and controllers as well as gate-arrays and ASIC's (Application Specific Integrated Circuits). The cost and turn-around time for producing some of these chips is very small.
There still remains a desire to produce faster and smaller digital chips, but the technology has yet to be discovered that will extend the range of these devices beyond their current capability. Since the availability of memory and microprocessors is abundant, efforts are being made to update interface technology. Data must be in digital form, hence the need for fast and efficient analog-to-digital converters as well as digital-to-analog converters. These comprise the information interface to the real world. In addition to the digital processing and data conversion elements, there is a need for more efficient and innovative sensors. Just as the digital industry sought more creative, cheaper designs, the analog and sensor designers did also.
The efforts of the digital industry made possible the complicated processing of silicon into an economically attractive technology. This industry has dictated the direction of both analog circuit and sensor designers. If a design can be implemented using conventional digital IC technology or by a slight modification, then it is advantageous to do so. This trend can be observed in the frequent use of CMOS technology in analog circuit design, though its origins were in digital design. In this decade, sensor designers have taken advantage of some of the unique processing aspects of silicon. The ability to precisely etch silicon into diaphragms and other structures makes possible total integration of sensor, interface, and information processing on one chip. The economic advantages of system integration are enormous.
A common type of sensor is the variable capacitance device. Capacitance of a capacitor is equal to .epsilon.A/d where .epsilon. is the dielectric constant, A is the area of the plates, and d is the gap thickness. Any change or combination of changes in the three variables can cause a capacitance change that can be detected. As an example, a pressure transducer can be formed by a diaphragm which supports one plate of the capacitor. Movement of the diaphragm changes the gap of the capacitor and thus its capacitance. As another example, shear force can be detected by lateral movement of one plate relative to another. That movement can vary the effective area of the capacitor. Further, the dielectric constant can be modified by chemical or thermal conditions or the like. Other capacitive sensors can detect tactile pressure or acceleration.
Unfortunately, sophisticated read-out circuitry must be employed to detect small capacitance changes in small capacitors due to the presence of large parasitic capacitances. The challenge is to design circuitry that is compatible with regular MOS fabrication and silicon micromachining processes and which can overcome the problems of parasitics and noise. Present systems employ both ac and dc techniques to sense capacitance change. However, the sense capacitors employed are much greater than the stray parasitics. As silicon diaphragms are scaled, new circuitry must be developed to handle smaller sense capacitors. This is due to the ability of semiconductor processes to scale horizontally across a wafer. If a structure can be built and be operational on a smaller scale, then it is more economical to do so since the cost of processing depends on area.